The present invention relates to a composite hybrid riser for carrying fluid between the seabed and a surface installation.
In present day oil production, several risers are arranged between the seabed and a surface installation such as a platform or a production ship. The risers may be either flexible or rigid. Lately, a number of concepts have been suggested, such as the concept shown in FIG. 1, in which a plurality of rigid risers are bundled together to form a hybrid riser 21, which is led up to a buoyancy member 22 near or at the sea surface 23. Fluids are transferred between the floating production unit 24 and the buoyancy member 22 through flexible risers 25, the advantages being that the buoyancy member and the couplings are maintained at a level where the effects of waves and wind are small, and the very expensive flexible risers, which are also subject to depth limitations, are only used for the transfer between the buoyancy member and the floating production unit. Such a solution has been disclosed in NO 159.546, which also includes a piping sheath enclosing the transport tubes.
However, the above solutions employ conventional steel tubing for transport of fluids, and tension members in the form of steel wires. This makes the riser very heavy, and leads to a requirement for a large buoyancy element. In water depths exceeding approximately 1000 metres, such risers can not be used, as the pipe wall thicknesses required due to the pressure are so large, as to make the riser so heavy that it would be practically impossible to obtain sufficient buoyancy. The tension in such a pipe would also exceed that which is possible to handle. In addition to the high buoyancy requirement, there would also be a requirement for a sturdy foundation on the seabed. Needless to say, building such a foundation at great depths is very costly.
Another disadvantage, which applies also at depths significantly less than 1000 metres, is associated with manufacture and tow-out of the riser. If the riser is manufactured onshore as one whole length or as long sections, for later tow-out to the installation site, the transportation itself will cause fatigue in the riser. Such fatigue shortens the expected working life of the riser, maybe by as much as 10%. The transportation distance therefore has to be as short as possible, thus limiting the number of possible manufacturing sites.
If the riser is to be put together from relatively short sections, which are transported on board for example a barge, the installation process will be made more expensive.
The present invention aims to provide a composite hybrid riser, which comprises those pipelines and cables normally needed between the seabed and a surface installation, and which does not or to a very much smaller degree exhibits the above mentioned disadvantages. This is achieved through the features stated in claim 1.
The present invention achieves a formidable weight saving, as components made from composite materials have a weight that is only a fraction of that of steel components. As a result of the weight saving, the requirements for buoyancy and foundation work are reduced.
Further, the fatigue life is increased to up to 10 times that of steel. Transportation to the installation site will therefore only cause a negligible reduction in the fatigue life, consequently the tow-out distance is not of critical importance, and the choice of manufacturing sites will be considerably greater.
Small dimension risers according to the invention may be coiled on the deck of a barge during shipment, thus simplifying transportation considerably.
A lighter and more flexible riser also makes installation easier. The capacity of cranes, winches and other equipment used may be reduced considerably. Installation may also be speeded up, due to the low weight and increased flexibility, and to the fact that the riser according to the invention tolerates a greater strain.